1. Field of the Invention
The present invention relates to forming a conductive interconnection between electronic devices. More particularly, the present invention relates to forming a conductive interconnection from a matrix material having ferromagnetic particles dispersed therein, wherein the ferromagnetic particles are aligned to make electrical contact between the electronic devices.
2. State of the Art
A flip chip is a semiconductor chip or die that has bumped terminations spaced around an active surface of the semiconductor die and is intended for face-to-face attachment to a carrier substrate or another semiconductor die. The bumped terminations of the flip chips are usually a "Ball Grid Array" ("BGA") configuration wherein an array of minute solder balls is disposed on an attachment surface of a semiconductor die, or a "Slightly Larger than Integrated Circuit Carrier" ("SLICC") configuration wherein minute solder balls are disposed on an attachment surface of a semiconductor die in an arrangement similar to a BGA, but having a smaller solder ball pitch (spacing) and diameter than a BGA.
The attachment of a flip chip to a substrate or another semiconductor involves aligning the solder balls on the flip chip with a plurality of contact points on a facing surface of the carrier substrate. Flip chip attachment requires the formation of contact terminals on flip chip contact sites, each consisting of a metal bond pad with a lead/tin solder ball disposed thereon. Flip chip attachment also requires the formation of solder joinable sites ("terminal pads") on the metal conductors of the carrier substrate, such as a printed circuit board ("PCB"), which terminal pads are patterned in a mirror-image of the solder balls arranged on the flip chip. A plurality of solder balls may also be formed on the facing surface of the substrate at the contact points. A quantity of liquid solder flux is often applied to the face of the chip and/or substrate, and the chip and substrate are subjected to elevated temperature to effect reflowing of the solder balls on the chip and/or corresponding solder balls on the substrate to effect electrical connection between the two components. The terminal pads on the substrate are usually surrounded by barriers of non-solderable/non-wettable material so that when the solder of the bond pads and of the chip contact sites melts and merges ("reflows"), surface tension supports the semiconductor chip by liquified solder columns above the substrate. After cooling, the chip is essentially welded face-down by these very small, closely spaced solder column interconnections. This connection technology is also referred to as "flip chip attachment" or "C4--Controlled Collapse Chip Connection."
High performance microelectronic devices generally comprise a number of flip chips, attached to a substrate or printed circuit board ("PCB") for electrical interconnection to other microelectronic devices. For example, a very large scale integration ("VLSI") chip may be electrically connected to a carrier substrate, such as a printed circuit board, or to other higher level packaging.
However, as electronic devices become smaller, the size requirements for semiconductors and the means for providing electrical connection between two semiconductors, between semiconductors and substrates, or between flexible circuits and rigid printed circuits become increasingly demanding by requiring precise electrical connections at extremely fine pitches. These demands have resulted in a growing interest in developing alternate methods for making electrical interconnections to replace solder balls. Among such alternate methods are the use of composite materials comprising conductive particles dispersed in a nonconductive material, such as polymers or resins, to form conductive bumps which are used as electrical interconnections in semiconductor structures between components bearing contact pads.
U.S. Pat. No. 5,258,577 issued Nov. 2, 1993 to Clements relates to a substrate and a semiconductor die with a discontinuous passivation layer. The discontinuities result in vias between the contact points of the substrate and the semiconductor die. A resin with spaced conductive metal particles suspended therein is disposed within the vias to achieve electrical contact between the substrate and the semiconductor die. U.S. Pat. No. 5,468,681 issued Nov. 21, 1995 to Pasch relates to interconnecting conductive substrates using an interposer having conductive plastic filled vias. U.S. Pat. No. 5,478,007 issued Dec. 26, 1995 to Marrs relates to using conductive epoxy as a bond pad structure on a substrate for receiving a coined ball bond on a semiconductor die to achieve electrical communication between the semiconductor die and the substrate.
Although the use of such conductive composite materials can achieve smaller sized and pitched interconnections, the conductive composite bumps require substantial quantities of conductive material dispersed therein. Stated another way, a relatively high percentage of conductive material, by volume, is required. Additionally, it is often difficult to achieve good electrical contact between the conductive composite bump and the contact pads because the polymers or resins used tend to surround the conductive material and curtail the ability to conduct between conductive particles. Thus, a good metal to metal interface may not be achieved by conductive composite bump.
Another method for making electrical interconnections is by the use of conductive films. These films may take a variety of forms and have been made having conductivity in one, two or three mutually perpendicular directions. With such films, it is customary to designate directions such that the x- and y-axis lie in the plane of the sheet or layer. Some such conductivity films have z-axis conductivity only, others have z- and y-axis conductivity, and isotropically conductive media have x-, y-, and z-axis conductivity (see, U.S. Pat. No. 4,923,739 issued May 8, 1990 to Jin et al.--two direction conductive sheet, and U.S. Pat. No. 4,548,862 issued Oct. 22, 1985 to Hartman--z-axis conductive tape).
Conductive films which can be used to replace solder bonding between two electronic components, as discussed above, are usually z-axis conductive films, wherein the z-axis conductive films may also physically bind the two electronic components together. Z-axis conductive films can be formed through a number of means, including dispersing ferromagnetic conductive particles throughout a viscous binder matrix, such as a non-conductive polymer or resin. Once the ferromagnetic conductive particles are dispersed, a magnetic field is applied to align the ferromagnetic particles into continuous conductive columns extending from a top surface of the binder matrix to a bottom surface of the binder matrix. Upon curing or otherwise hardening, the binder matrix will become an adhesive layer, an elastomeric layer, or another type of dielectric material, depending on the binder matrix used. Where electrical connection on a very fine pitch is required, the conductive columns may be placed only where the contact sites (e.g., terminal pads) are located, typically requiring indexing (i.e., precisely aligning) the conductive film to align the conductive columns with the contact sites, or the conductive columns may be formed with such close spacing, relative to the spacing of the contact sites, that indexing is not required.
Although conductive films have their advantages, having to index the conductive film is a time-consuming (and thus expensive) process. Furthermore, the fine pitched, non-indexed conductive films waste a great deal of conductive material, since only a small percentage of conductive columns in the conductive films actually perform the function of making the electrical contacts between the two electronic components.
Therefore, it would be advantageous to develop a technique for making an electrical interconnection between two electronic components which overcome the above disadvantages, while using inexpensive, commercially-available, widely-practiced semiconductor device fabrication techniques and apparatus without requiring complex processing steps.